Melbourne Metropolitan Area and Environs in September 2010: Cool and cloudy with slightly less rainfall than usual

Cool days

Average rainfall

Normal nights

Summary

Across the Melbourne Metropolitan Area, September 2010 was a cool and cloudy month, with slightly below average precipitation at most places. This was on account of persistent southwesterly flow with high pressures centred to the south of Western Australia bringing cool, moist air to Victoria.

The cloud acted as a "blanket" and overnight temperatures, therefore, were generally near to those usually experienced. However, the southern origin of the airmass meant that daytime temperatures at most localities were cooler than
those registered in the past. A major frontal passage early in the month, resulted in a very wet couple of days in Melbourne, but total
rainfall for September was generally slightly less than that usually recorded, on account of the subsequent frontal passages being relatively weak. The air following the front which moved across Victoria on the 28th was extremely cold, leading to snow flurries and near-zero temperatures on the Dandenongs during the morning of the 29th.

Details

Rainfall

High rainfalls were recorded in the first week of the month, with several stations registering daily totals of 10 to 20mm on the 4th. In fact, this was the wettest single day in the Central district, with 25.0 mm recorded at Avalon Airport.

Melbourne Airport was the driest location in September receiving just 40.2 mm, 6 mm below average. Melbourne Regional Office was also below its climatological average of 58 mm, observing just 49.6 mm. In fact, approximately 40 mm of this total was received in the first week of the month. Weak frontal systems meant that drizzle and weak rain accompanied persistent cloud cover but did not result in substantial rainfall totals.

Avalon Airport, Coldstream and Ferny Creek were the only stations to record slightly above average rainfall this month. In contrast, Scoresby and Bundoora were particularly low, receiving 30% less rainfall than usual for September.

Temperature

Maximum temperature

Warm days were few and far between this month, with maximum temperatures across the Central district, on average, more than 1.5°C below normal. Overcast conditions and cool, moist south-westerly flow frequently kept daytime temperatures below 16°C. Several sites also recorded their lowest average daytime September temperatures since the early 1990's this month.

A slow moving high pressure system brought sunny spring-like conditions to Melbourne for the AFL Grand final weekend, with 22.1°C recorded at the Melbourne Regional Office on the 26th. This ended a long stretch of sub 20°C temperatures at the Melbourne Regional Office; with 138 consecutive days making it the fourth longest such stretch since 1855.

Interestingly the coldest day at the Melbourne Regional Office this month was the 1st, reaching just 13.3°C after a mild night (11.1°C). This occurred as a result of the passing of a cold front which brought widespread rainfall and cloudy conditions.

Minimum temperature

Night-time temperatures were very close to average with frequent cloud cover preventing significant overnight cooling. The coldest nights were the 3rd, with clear skies bringing temperatures close to zero at Coldstream, and the 8th when most of the Central district experienced frosty temperatures of just 2°C.

The warmest nights on average at 9.1°C were experienced at the Melbourne Regional Office which was over 1°C lower than normal. This site, not surprisingly, also observed the warmest night of the month (12.8°C) for the Central district on the 27th.

Wind gusts

The strongest wind gust was 102 km/h at Laverton RAAF on the 5th, associated with the intensification of the tropical low pressure system which caused widespread flooding in northern Victoria.

Notes

A Monthly Climate Summary is prepared to list the main features of the weather in Melbourne Metropolitan Area and Environs using the most timely and accurate information available on the date of publication; it will generally not be updated. Later information, including data that has had greater opportunity for quality control, will be presented in the Monthly Weather Review, usually published in the fourth week of the month.

This statement has been prepared based on information available at
11 am on Friday 1 October 2010.
Some checks have been made on the data, but it is possible that results will change
as new information becomes available.

Averages are long-term means based on observations from
all available years of record, which vary widely from site to site.
They are not shown for sites with less than 10 years of record, as they cannot then be calculated reliably.
The median
is sometimes more representative than the
mean
of long-term average rain.

The Rank indicates how rainfall this time compares with the climate record for the site,
based on the
decile ranking
(very low rainfall is in decile 1, low in decile 2 or 3,
average in decile 4 to 7, high in decile 8 or 9
and very high is in decile 10).
The Fraction of average shows how much rain has fallen this time as a
percentage of the long-term mean.